1. Field of the Invention
The present invention relates to fast neutron sensing and dosimetry using a proton radiator coated on a transparent glass phosphor that exhibits one or more of the properties of thermoluminescence, optically stimulated luminescence and scintillation. More particularly, the invention relates to the use of a neutron detector in a remote sensing unit for fast neutrons and a fast neutron personal dosimeter. The radiation dose to the detector is determined by monitoring one or more of the following phosphor characteristics: laser heated thermoluminescence, optically stimulated luminescence and scintillation.
2. Description of the Background Art
Fast neutron dosimetry is important in both military and civilian nuclear applications. Fast neutron dosimetry is required for the protection of personnel employed at civilian nuclear power generating plants as well as civilian nuclear reprocessing and enrichment plants. Fast neutron dosimetry is also required for the protection of Department of Defense personnel involved with the operation of nuclear propulsion systems and the handling of nuclear weapons. The U.S. Navy has been actively searching for a simple and effective technique that provides an accurate measure of the absorbed radiation dose of fast neutrons. The Navy's radiation protection needs for x-rays and thermal neutrons have been satisfied for the past quarter century using solid state thermoluminescence dosimetry (TLD). Thermoluminescence dosimetry is a well established technology used extensively for environmental and personnel monitoring of radiation exposure. At the present time, it is the method of choice for all dosimetry applications in the U.S. Navy, supplanting the use of conventional film badges.
The thermoluminescent dosimetric materials in current use include a variety of doped crystalline phosphors that form long-lived electron and hole traps upon exposure to radiation. The release of the trapped electrons and/or holes upon heating with the simultaneous release of a photon is known as thermoluminescence. Traditional thermoluminescent dosimeters use a doped crystalline phosphor in the form of powders or extruded chips which are typically opaque and highly light scattering. Examples of these materials would include polycrystalline powders of lithium fluoride or calcium fluoride each doped with activators, such as titanium, magnesium and phosphorus, and powders of calcium fluoride doped with manganese. The phosphor grains are micron sized and are highly light scattering, thus reducing the brightness of the phosphor emission and placing practical limitations on the thickness of the phosphor.
A common technique used to monitor human exposure to fast neutrons is the use of nuclear track detectors. These detectors operate by detecting the tracks of recoil protons produced by the elastic scattering of fast neutrons in a hydrogen-rich material. Solid state track detectors, such as the plastic CR-39, are hydrogen-rich but require complex processing, including chemical etching to detect the neutron dose. Such detectors may also be coated with polyethylene as a proton radiator, which has a greater mole fraction of hydrogen than the track detector to improve the track detector's sensitivity. A problem with these track detectors is the uncertainty in discriminating tracks due to protons from imperfections or abrasions in the plastic.
Electronic silicon diode dosimeters with polyethylene proton radiators have also been used for neutron dosimetry. The silicon diode dosimeters are subject to several problems including the lack of long term stability, thus requiring recalibration before every use. In addition, electronic dosimeters often work well only in steady neutron fields and do not work well in pulsed or erratic fields and generally do not yield accurate results with intermediate or low energy neutron fields. Moreover, they are active devices requiring an external power supply.
Silicon diode sensors have also been used as remote fast neutron sensors. The silicon diode sensors are limited to cable lengths of about twelve feet and cannot be placed tens or hundreds of meters from the control electronics. For long distance applications, silicon diode sensors are manufactured with radio transmitting capabilities which significantly increase the complexity and cost of the sensor.
Bubble dosimeters have also been used for fast neutron dosimetry. These devices are interesting because of their complete insensitivity to gamma radiation, however, they are also expensive, lack dynamic range and a have limited useful lifetime.
Until the present invention, thermoluminescent detectors have not been suitable for fast neutron dosimetry, however, thermoluminescent detector phosphors doped with .sup.6 Li or B are used for thermal neutron dosimetry. The problem with using conventional thermoluminescent phosphors, including laser heated ones, is that the phosphor must be heated to high temperatures to release the trapped charges and measure the radiation dose. These high temperatures destroy the polyethylene proton radiator rendering it unsuitable for fast neutron dosimetry. Use of ultraviolet stimulated luminescence of thermoluminescent phosphors has also been reported. Powdered thermoluminescent phosphors were mixed with polyethylene and pressed into dosimeter disks. After exposure to high energy neutron radiation, the dosimeter disks were heated to a temperature below the melting point of the polyethylene and exposed to ultraviolet light to efficiently stimulate luminescence. Problems with this technique include scaling since only luminescence from the surface is detected. The dosimeter also cannot be reused because it cannot be reannealed at high temperatures to depopulate all the trapped charges.
Measurement of fast neutron radiation levels in hazardous areas such as nuclear waste sites, or inaccessible areas, such as sampling wells for in-situ ground water monitoring, requires the development of sensitive remote radiation measurement techniques. Remote sensing using fiber optics would be a preferable solution because of the small size of the fibers, their mechanical flexibility, and optical, rather than electrical, signal transmission. Several fiberoptic-based radiation sensing systems have been described that utilize radiation induced changes in the optical characteristics of the fiber, such as darkening of the glass, optical phase shifts due to heating, or change in the birefringence of a polarization maintaining fiber. Another approach utilizes a traditional thermoluminescent phosphor attached to the end of a multi-mode fiber optic cable. A laser is used to heat an adjacent absorber and the absorber then heats the thermoluminescent phosphor by thermal diffusion.
Each of the described fiber optic-based techniques has serious shortcomings that limit practical applications. The measurement of radiation induced darkening is straightforward but is limited in both sensitivity and dynamic range. In addition, photo-darkening is usually the result of photochemical damage and is irreversible. Phase shift measurement requires the use of an interferometer with phase sensitive detection and feedback controls, long fiber lengths, complex signal processing techniques and is extremely sensitive to changes in ambient temperatures. The fiber optic, laser heated thermoluminescent approach requires the use of a very thin layer of phosphor material because of inefficient heating of the phosphor by thermal diffusion and, additionally, problems associated with light scattering. The limitation on the thickness of the thermoluminescent phosphor material ultimately limits the sensitivity of this method.
The aforementioned problems in the prior art as well as others not specifically mentioned have been overcome by the present invention as described hereinafter.